In satellite communications technology, maintenance-free refrigerator-cooled electronic systems (antennas, high temperature superconducting filters, amplifiers) are already being used and will be used increasingly in the future. Such systems must be constructed to provide a service life of several years. The refrigerators are subject to wear in operation and, therefore, in order to avoid a system failure, at least one other refrigerator must be provided as a redundancy. During operation, this redundant refrigerator must be thermally isolated from the end use device or application, since it otherwise acts as a parasitic heat bridge, and must be thermally coupled with the application when the primary cooling refrigerator fails. At the same time, the defective primary refrigerator must be thermally isolated from the end use device or application. The switch-over to the redundant refrigerator must occur as quickly as possible so that the end use device or application does not heat up excessively in the meantime and possibly have to be temporarily taken out of operation. For this reason it is advantageous to cool down the redundant cooler or refrigerator in a no-load condition and only then connect it to the end use device or application as soon as its cold head temperature is lower than the temperature of the application.
In order to solve this problem, various conventional devices have been used, such as active electromechanically or pneumatically operated heat switches, self-triggering uni-directional cryogenic heat exchanger tubes or heat pipes (U.S. Pat. No. 4,673,030), or gas gap heat flow switches that are pumped by cryosorption (U.S. Pat. No. 4,771,823).
The active mechanical systems have provided the best solution functionally to date. They are, however costly to manufacture, require additional control electronics, and themselves carry a significant risk of failure.
Uni-directional cryogenic heat pipes are still the subject of intensive development. They are very costly to manufacture, as either high pressure engineering or cryo-engineering is required to fill them with the working medium. At ambient temperature they are subject to a high internal pressure, which requires great tube wall thicknesses. Consequently, these heat pipes have a poor switching ratio and are still perceptible as parasitic heat bridges even after isolation or separation of the heat contact.
In contrast, it is substantially less costly to manufacture gas gap heat flow switches that are pumped by cryosorption. These switches are also self-actuating without an additional control. In order to switch over to the redundant refrigerator, however, the application and the failed refrigerator must be heated to the extent that the cryosorption pump desorbs sufficient gas to close the heat flow switch of the redundant refrigerator. A high-temperature superconducting application would have to be taken out of operation to do this.